(A) Field of the Invention
This invention relates to comminuting utilizing disc refiners and especially relates to the formation of wood pulp by comminuting wood chips.
(B) History of the Prior Art
Ground wood for use as wood pulp is currently being produced by two different methods. One comprises the fiberization of wood logs by grinding with a rotating stone within an abrasive surface, while in the other method, wood is first reduced to chips and then fiberized in disc refiners. Depending upon the desired quality of ground wood, 80 to 100 horsepower days per ton of power is consumed in the production of ground wood using either the stone or the disc refiner method. However, only a fraction of this energy is used for the liberation of the fibers from the wood log or chip; the rest is dissipated as heat or lost in other ways.
It is also known that hard woods do not yield an acceptable quality of ground wood, at least partly because of the greater slenderness of their fibers in comparison with the fibers of soft woods and the higher density and brittleness of the hard woods. Because of this brittleness and small fiber diameter, the particles torn out of the hard wood log by the stone grinder or shaved off from the chips by the bars of the disc refiner plates tend to form rather coarse particles containing layers of many fibers as well as very small particles forming "fines."
One type of mechanical pulp is made in disc refiners under pressure, the product being known as thermo or thermal mechanical pulp (TMP). This process allows the production of mechanical pulp of much higher quality than other mechanical pulping methods, but it consumes 30 to 50 percent more energy than standard methods.
Thermal mechanical pulp is made from wood chips a majority, having a size in maximum dimension of from 3/8 to 1 inch. In conventionaloperations, plates of various breaker bar patterns are mounted on the solid backing disc of the refiners, which rotate in opposite direction and by attrition reduce the wood chips to a coarse fibrous mass. The product of this first stage is not yet ready for paper production and has to be refined once more in a second stage which is also a disc refiner. This time the refining is done under atmospheric pressure to achieve the final quality. The grinding surface of the discs of the prior art all consist essentially of breaker bars arranged in various patterns.